1. Field of the Invention
The present invention relates to an improved high-voltage fuse cutout and, more particularly, to an improved high-voltage fuse cutout which exhibits improved operating performance. The high-voltage fuse cutout of the present invention is an improvement over the inventions claimed in commonly assigned U.S. Pat. No. 2,816,879 issued to Lindell and Application Ser. No. 132,922 filed Mar. 24, 1980 in the name of Schmunk. An object of the present invention is to permit the use of standard fuse links and standard fuse cutouts for interrupting high and low level fault currents at higher circuit voltages than hitherto achievable.
2. Discussion of the Prior Art
Single-vented fuse cutouts of various types are well known. A typical single-vented fuse cutout includes a hollow insulative fuse tube with a bore therethrough and conductive ferrules mounted to the opposite ends thereof. Typically, the bore is lined with an ablative arc-extinguishing material, such as horn fiber, bone fiber, vulcanized fiber, or the like. One ferrule (often called the exhaust ferrule) is located at an exhaust end of the bore and usually includes a trunnion casting which interfits with a trunnion pocket of a first contact assembly carried by one end of a porcelain or cycloaliphatic epoxy insulator. The other ferrule is normally held and latched by a second contact assembly carried by the other end of the insulator so that the fuse tube is normally parallel to, but spaced from, the insulator. The contact assemblies may also be respectively carried by two separated insulators mounted to a common base. In this latter arrangement, when the other ferrule is normally held and latched by the second contact assembly, the fuse tube is normally perpendicular to the insulators. In either arrangement, the insulators are mountable to a crossarm of a utility pole or a similar structure.
A fuse link is located within the fuse tube bore with its ends respectively electrically continuous with the ferrules. One point of an electrical circuit is connected to the first contact assembly, while an opposed point of the circuit is connected to the second contact assembly. Usually, the fuse tube is oriented generally perpendicular to the ground so that the exhaust ferrule and the first contact assembly are located below the other ferrule and the second contact assembly.
The fuse tube may include a high-burst strength outer portion--for example, a fiberglass-epoxy composite--surrounding the arc-extinguishing material lining the bore thereof. That the arc-extinguishing material is ablative means that it decomposes into gaseous components inimical to arcs when exposed to the heat thereof.
Normal currents in the electrical circuit flow without affecting the fuse link. That is, the I.sup.2 t heating effect of normal currents is insufficient to fuse, melt, vaporize, or otherwise render discontinuous the fuse link. Should a fault-current or other overcurrent to which the fuse link is designed to respond occur in the circuit, the fuse link operates as described below. Operation of the fuse link permits the upper ferrule to disengage itself from the upper contact assembly, whereupon the fuse tube rotates downwardly due to coaction of the trunnion casting and the trunnion pocket. If the cutout operates properly, current in the circuit is interrupted and the downward rotation of the fuse tube both gives a visual indication that the cutout has operated to protect the circuit and produces an air gap between the contact assemblies which is capable of continuously withstanding the circuit voltage.
Typical fuse links include a first or upper terminal and a second or lower terminal between which there is normally connected a fusible element made of elemental silver, silver-tin, or the like. Also connected between the terminals may be a strain wire for a purpose described below. The upper terminal is electrically continuous with, and is usually mechanically connected to, a button contact assembly which is engageable by a portion of the upper ferrule on the fuse tube. The lower terminal is connected to a flexible stranded length of cable. A sheath surrounds at least a portion of the lower terminal, the fusible element, the strain wire (if used), the upper terminal, and some portion of the flexible stranded cable. The sheath is typically a cellulosic material impregnated or coated with an ablative arc-extinguishing material (such as boric acid, magnesium borate, or the like) or may be made almost entirely of an ablative arc-extinguishing material such as horn fiber. Such ablative arc-extinguishing materials are well known and comprise compounds or compositions which, when exposed to the heat of a high-voltage arc, decompose to rapidly evolve large quantities of de-ionizing, turbulent and cooling gases. Typically, the sheath is shorter than the fuse tube bore and terminates well short of the exhaust end thereof.
The free end of the stranded cable extends from the exhaust end of the bore and is tensioned by a spring-loaded flipper on the trunnion casting. The tension exerted on the cable by the flipper attempts to pull the cable and the lower terminal out of the sheath and out of the fuse tube. The tension force applied by the flipper is normally restrained by the strain wire, many fusible elements not having sufficient mechanical strength to resist this tension.
In the operation of typical cutouts, a fault current or other overcurrent results, first, in the melting or vaporization of the fusible element, followed by melting or vaporization of the strain wire. Following such melting or vaporization, a high-voltage arc is established between the terminals within the sheath, and the flipper is now free to pull the cable and the lower terminal out of the sheath and, ultimately, out of the fuse tube. As the arc forms incident to a low level fault current, which may here be taken to be below about 1000 amperes, the arc-extinguishing materials of the sheath decompose and larger quantities of de-ionizing, turbulent, and cooling gases are rapidly evolved. As the arc forms at high fault currents, which for purposes here may be taken to be currents in excess of about 1000 amperes, the violence of the arc causes the sheath to burst and permits the arc to interact with the arc-extinguishing materials of the bore of the fuse tube which decompose and evolve even larger quantities of de-ionizing, turbulent, and cooling gases. In either event, the movement of the lower terminal under the action of the flipper, and the subsequent rapid movement thereof due to the evolved gases acting thereon as on a piston, result in elongation of the arc. The presence of the de-ionizing, turbulent, and cooling gases plus arc elongation may, depending on the level of a fault current or other overcurrent, ultimately result in extinguishment of the arc and interruption of the current at a subsequent current zero. The loss of the tension on the stranded cable originally applied by the flipper permits the trunnion casting to experience some initial movement relative to the exhaust ferrule, which in turn permits the upper ferrule to disengage itself from the upper contact assembly. This initiates the downward rotation of the fuse tube and its upper ferrule to a so-called "drop out" or "drop down" position.
Typically, the fuse tube bore has a constant circular cross-section and is just large enough to accommodate the insertion thereinto of the fuse link. In typical single-vented fuse cutouts, placement of the fuse link in the fuse tube bore closes the end thereof near the upper ferrule, but the exhaust end of the bore remains open. As noted earlier, it is through this exhaust or open end of the bore that the cable of the fuse link extends.
Improper operation of fuse cutouts and typical fuse tubes thereof as described above has been detected. Specifically, at or near the maximum interrupting current rating of the above-described cutouts, improper current interruption or failure to interrupt current has been detected. An examination of typical cutouts and their fuse tubes, both during and after attempts at operation, has led to the conclusion that gas evolved deep in the bore--that is, remote from the exhaust end and whether or not evolved from the sheath or from the walls of the bore--often stagnates, that is, is prevented from efficiently exiting from the exhaust end of the bore. It has also been observed that the lower terminal of the fuse link may partially block the exhaust end of the bore; this partial blockage exacerbates the stagnation of gas evolved deep within the bore. It has been postulated that the stagnation of gas evolved deep within the bore due to arcing before a current zero occurs prevents recovery of sufficient dielectric strength within the bore at the current zero, thus preventing effective and permanent current interruption. In general, the problem of gas stagnation within the bore has been solved by the invention claimed in the '922 application. Specifically, as claimed in that application, the bore of the fuse tube is mildly tapered at the exhaust end so as to have a smaller diameter closer to the first end of the fuse tube and a greater diameter at the exhaust end of the fuse tube. The amount of the mild taper is sufficient to prevent the stagnation of the gases within the bore. The mild taper may be either a smooth taper or a series of steps in the wall of the bore. The included angle of the mild taper measured between the exhaust end and the inception of the taper is from about 1.degree. to about 3.degree.. As disclosed in that application, the specific amount of taper may be varied depending upon the maximum current interruption rating of the fuse cutout and the voltage at which it is intended to be used.
As noted earlier, fuse cutouts may be called upon to interrupt both low and high level fault currents. The separation of the terminals of the fuse link within the sheath, the action of the sheath itself, and the interrelationships among the fuse tube, the sheath and the lower terminal have been found to be the primary factors responsible for low fault current interruption. Specifically, at low fault currents, if the sheath does not burst or rupture and remains integral, the arc between the terminals is elongated entirely therewithin. The elongating arc interacts with the arc-extinguishing material of the sheath, evolving the arc-extinguishing gases as described above. It is postulated that if sufficient arc-extinguishing gas is evolved from the sheath and if the pressure of this gas within the sheath remains sufficiently high at a current zero while the lower terminal is still within the sheath, there will be sufficient dielectric strength between the terminals due to the presence of the arc-extinguishing gas to prevent reinitiation of the arc. The gas pressure within the sheath depends on a number of factors including the level of the fault current, the relative sizes of the lower terminal and the sheath diameter and the length of the sheath. It has further been found that a sufficient length of sheath to achieve low fault current interruption is substantially shorter than the length of the fuse tube bore, regardless of the voltage at which the fuse cutout is used.
The fuse cutout may also be called upon to interrupt high fault currents. As noted earlier, at high fault currents the sheath usually ruptures and the extinguishment of the arc formed and elongated between the terminals of the fuse link is primarily due to the evolution of the arc-extinguishing gas from the bore of the fuse tube. The length of the fuse tube is initially determined by the voltage at which the fuse cutout is used; higher voltages, of course, require longer fuse tubes. It has been found that, if the arc forming between the separating fuse link terminals is elongated by a distance approaching the length of the fuse tube bore, the pressure generated in the fuse tube may rupture it. It has also been found that high fault current interrupting capability of a given fuse tube may be maximized by substantially reducing the length of the arc elongated between the fuse link terminals to an optimum length which is substantially less than the length of the fuse tube.
The '979 patent illustrates a fuse cutout in which the length of the arc produced and elongated during high fault current interruption is limited. Specifically, in that patent, the entire fuse link including its terminals, its fusible element, and its sheath is positioned not at the upper end of the fuse tube, but somewhat closer to the exhaust end of the fuse tube than hitherto described. This positioning is achieved by a so-called arc-shortening rod, which is mounted within the fuse tube bore and is mechanically and electrically connected to the upper ferrule and to the upper terminal of the fuse link. The fuse link and the sheath are positioned closer to the exhaust end of the fuse tube by a distance equal to the length of the arc-shortening rod. The fuse tube is, nevertheless, sufficiently longer than the sheath so that the entire sheath is contained within the bore of the fuse tube. Upon the occurrence of low current faults, the arc produced between the terminals of the fuse link is elongated within the sheath, which does not burst, and sufficient arc-extinguishing gas is evolved to effect extinguishment thereof. Upon the occurrence of higher fault currents, the amount of the fuse tube bore with which the elongating arc can interact following rupture of the sheath is decreased due to the initial positioning of the fuse link terminals closer to the exhaust end of the fuse tube.
While the use of the arc-shortening rod has been found to improve high fault current interruption, low fault current interruption can put a greater burden on the fuse link. Specifically, the production of a sufficiently high pressure within the sheath is enhanced by locating the fuse link at or near the closed end of the fuse tube bore. The closer the fuse link is to the exhaust end of the bore--as is the case where the arc-shortening rod is used--the less likely it is that sufficient pressure to assure arc extinguishment will be present within the sheath. Simply stated, during fuse link operation, there is more back pressure at the upper end of the bore than there is at or near the exhaust end. The amount of such back pressure is at least partly responsible for the level of gas pressure within the sheath. Further, higher back pressure within the bore enables the sheath to more easily resist bursting at low fault current levels. As noted earlier, at some high fault current level the sheath will burst; but it is desirable for the sheath to remain integral over as broad a range of low fault current levels as possible. Thus, the closer the fuse link is to the closed end of the bore, the higher will be the level of the low fault current which will burst the sheath. In order to optimize the operation of fuse cutouts utilizing arc-shortening rods, it has been found necessary to use standard fuse links with optimized designs, that is, fuse links which embody improved characteristics. An improved fuse link may include one or more of the following: (a) a sheath having a higher burst strength, (b) a stronger bond between the sheath and the upper terminal, which mounts the sheath, (c) careful dimensioning of the lower terminal and of the diameter and length of the sheath and improved manufacture of the cable to assure both free, unimpeded movement of the lower terminal through the sheath and the production of an optimum pressure within the sheath which will not burst the sheath but will result in arc extinguishment.
It would be desirable to be able to use a less costly standard fuse link in a fuse cutout at a variety of circuit voltages and to have the cutout effectively operate to interrupt both low fault currents and high fault currents. Accordingly, a general object of the present invention is to provide an improved fuse cutout which utilizes a fuse tube of the minimum length required, and in which low current faults are effectively interrupted by the evolution of arc-extinguishing gas from the sheath while high current faults are effectively interrupted by the evolution of arc-extinguishing gas from the fuse tube bore while at the same time preventing rupture of the fuse tube.
As can be seen from the above discussion, the pressure of gas evolved during arc elongation can be either too high or too low. At high fault currents a pressure which is too high may burst or fracture the fuse tube, thus requiring its replacement and--more importantly--probably compromising the ability of the cutout to successfully interrupt fault current. The arc-shortening rod of the '979 patent ameliorates, to some extent, the pressure-producing effect of high fault current arcs, as does the mild taper of the '922 application. At low fault current levels, if the pressure within the sheath is too low, current interruption may not occur. The arc-shortening rod of the '979 patent may position the sheath too close to the exhaust end of the bore so that the pressure does not get sufficiently high. Moreover, the lower back pressure within the bore at the location of the sheath dictated by the arc-shortening rod may render the sheath vulnerable to bursting at current levels below those where bursting is desirable. Further, the taper of the '922 application may prevent the production of sufficient pressure within the sheath at very low fault currents.
Accordingly, another general object of the present invention is the combination in a fuse cutout of the desirable functions of the arc-shortening rod (preventing the bursting of fuse tubes at high fault current levels) and the mild taper (preventing stagnation within the bore) without compromising the operation of the cutout at lower fault current levels. A further object of the present invention is the achievement of the last-stated ends and the widening or broadening of the range of fault current levels which may be interrupted by a fuse cutout using a standard fuse link.